1. Field of the Invention
This invention relates to catalysts supported on metal structures of superior resistance to corrosion at both high and low temperatures and to methods for producing same. This invention also relates to a method for preparing non-porous stable oxide coatings on metal surfaces. In another aspect the present invention relates to a method for producing metal articles resistant to acidic corrosion at ambient temperatures. In still another aspect this invention relates to a method for preparing metal surfaces for coating with catalytic agents.
2. Brief Description of the Prior Art
Ceramic monolith structures are well known in the art as structural supports for a high surface area metal oxide layer, typically gamma alumina, on which is impregnated a suitable catalytic agent such as a noble metal like platinum. Because the gamma alumina is applied as a slip or slurry of alumina particles in an aqueous solution to a multichannel honeycomb, the honeycomb channels must be sufficiently large to permit flow through and removal of relatively viscous slurries. Another problem is that ceramic structures are subject to mechanical and thermal shock damage which can damage high surface area oxide layers. In addition, at moderately high temperatures the high surface area oxide layer can react with the ceramic support structure resulting in loss of catalyst activity. Consequently, there has been considerable interest in the use of metal honeycomb structures as catalyst structures. An important advantage of metal honeycombs is that the structure walls can be made much thinner than the walls of ceramic honeycombs. This allows for a greater open area in the flow direction for a given number of flow channels per unit area. Typically, such honeycomb catalysts are prepared by conventional slip coating methods on a suitably prepared metal surface, as described for example in the U.S. Pat. No. 4,188,309. Regardless of how the honeycomb structure is prepared for slip coating, application of a surface coating of a catalyst support significantly increases wall thickness, at least partially cancelling the benefits of having thin structure walls. A second disadvantage is that the thermal expansion differences between a metal and an oxide slip coating can result in spalling off of the relatively thick slip during thermal cycling. Consequently, in spite of the man potential advantages of metal monoliths as catalyst structural supports, metal monolith honeycomb catalysts have not heretofore displaced conventional ceramic honeycomb catalysts.
The U.S. Pat. No. 4,188,309 describes the state of the art as of 1978 and is incorporated herein by reference thereto.